Previously, we were comparing the signed int `target_length` (returned by WideCharToMultiByte) to a size_t string length, resulting in a signed/unsigned comparison warning as follows:

```
windows\runner\utils.cpp(54,43): warning C4018:  '>': signed/unsigned mismatch
```

WideCharToMultiByte returns:
* 0 on error
* the number of bytes written to the buffer pointed to by its fifth parameter, lpMultiByteStr, on success.

As a result it's safe to store the return value in an unsigned int, which eliminates the warning.

No changes to tests since this is dependent on end-user project settings/modifications and does not trigger a warning with default project settings.

Fixes: https://github.com/flutter/flutter/issues/134227

To implement windows-arm64 support, it is needed to add architecture as a subdirectory (https://github.com/flutter/flutter/issues/129805).

In short, when performing a flutter windows build, we have:
- Before: build/windows/runner/Release/gallery.exe
- After: build/windows/x64/runner/Release/gallery.exe

This convention follows what flutter linux build does.

Addresses: https://github.com/flutter/flutter/issues/129805
Addresses: https://github.com/flutter/flutter/issues/116196

Design doc: [flutter.dev/go/windows-arm64](https://flutter.dev/go/windows-arm64)

This app will be use for a11y assessments. See [design doc](http://go/flutter-gar-reporting-q3-2023)    (internal only)